1. Field of the Invention
This invention relates to mobile radio systems and more specifically to transmitting, receiving and demodulating digital information in a digital radio system.
2. Description of Related Art
In digital mobile radio systems, base units communicate with mobile radio units. Both base and mobile units employ a transmitter and receiver. During communication, one of the communicating units transmits symbols embedded in a continuous radio-frequency (RF) signal allowing a receiver to decode the RF signal into digital information. The motion of the mobile units causes impairments in the signal, or channel impairments, known as Doppler shifts.
Transmitted signals may be received directly by the receiving unit as a first ray, and also as a second ray which is reflected from a physical object. This is a channel impairment known as multi-path propagation. The second ray may arrive at the receiving unit slightly later due to its longer transmission path. This is known as delay spread.
Another channel impairment is known as intersymbol interference (ISI). Data symbols are usually filtered before transmission. This filtering causes a symbol to have a duration that is longer than a symbol duration before filtering. ISI arises when a symbol overlaps onto a subsequent symbol period and thereby affects the next transmitted symbol. ISI arises due to multi-path propagation and due to the nature of a filtered transmitted symbol.
If the above channel impairments remain uncorrected, the received signal when demodulated could result in data with a high probability of error, causing a significant increase in bit error rate (BER) in the decoded digital information. In order to improve the BER, an equalizer is employed to demodulate the ISI-impaired received signal.
For the U.S. digital cellular system (IS-54), a time division multiple access (TDMA) system, the following mobile channel model has been specified in "Recommended Minimum Performance Standards for 800 MHz Dual-Mode Mobile Stations", (incorporating EIA/TIA 19B), EIA/TIA Project Number 2216, March 1991 for the purpose of evaluating the performance of candidate equalizer designs. The mobile channel is specified to be a two ray multipath model. Both rays are independently Rayleigh faded, with equal average power, and frequency shifted by a Doppler spread corresponding to the vehicle speed. The delay spread is defined in terms of a delay interval (.tau.) which is the difference in microseconds between the first and last ray in the two ray channel model.
Any communications system for U.S. Digital cellular telephones must meet the specified requirements on the channel model described above. Further, it is desirable that the mobile transmitter/receiver be small, (preferably hand-held), have low power consumption and be of low complexity. The need for an equalizer in a cellular radio telephone increases the complexity and power drain. Hence there is a need to design the equalizer for the above-mentioned application such that it is of low power, low complexity and will meet the performance specifications of the U.S. digital cellular TDMA standard.
Non-linear equalization schemes such as decision feedback equalization and equalization using maximum likelihood sequence estimation (MLSE) are considered appropriate for the above mobile channel. Decision feedback equalizers (DFEs) present a powerful equalization scheme as described in "Decision Feedback Equalization for Digital Cellular Radio" by S. Chennakeshu, A. Narasimhan, J. B. Anderson, Proceedings of ICC,339.41-339.4.5, pp. 1492-1496,1990; and "An Adaptive Lattice Decision Feedback Equalizer for Digital Cellular Radio", Proceedings of VTC, pp. 662-667, 1990 by A. Narasimhan, S. Chennakeshu, J. B. Anderson and can be very effective when used in conjunction with antenna diversity. However, they are usually too complex to implement in a mobile receiver. The complexity is mainly due to the requirement for a fast recursive least squares (FRLS) algorithm to estimate and track the channel impulse response (CIR). Further, these approaches are prone to error propagation and consequently high BER at higher vehicle speeds.
A DFE approach with a novel block-adaptive strategy is described in "Adaptive Equalization and Diversity Combining for a Mobile Radio Channel" by N. W. K. Lo, D. D. Falconer, A. U. H. Sheikh, Proceedings of Globecom, 507A.2.1-507A.2.5, pp. 923-927, 1990 wherein the time varying CIR estimates are computed through interpolation between known CIR estimates. This scheme entails lower complexity than the above approaches and eliminates the effect of error propagation, since there is no decision directed estimation of the CIR. When used in conjunction with antenna combining this method has been shown to be very effective. However, in the U.S. digital cellular system the use of .pi./4-shifted DQPSK modulation causes the CIR estimates to have phase ambiguities, which precludes direct application of the block-adaptive CIR interpolation scheme.
Another problem with the DFEs is that they exhibit sensitivity to the non-minimum phase condition in the channel, which occurs when the second ray is stronger than the primary ray. These limitations of the DFE make the MLSE based algorithms preferable for use in mobile radio receivers.
An MLSE based receiver which may be used in U.S. digital cellular system is disclosed in U.S. Patent Application "Adaptive MLSE-VA based Receiver for Digital Cellular Radio" by S. Chennakeshu, A. Narasimhan, J. B. Anderson Ser. No. 07/753,578 filed Sep. 3, 1991, and now U.S. Pat. No. 5,285,480. This MLSE based receiver is also relatively complex to implement.
Analysis of the equalizer algorithm indicates that the complexity stems from i). the branch metric computations, ii).CIR estimation and tracking which requires an adaptive algorithm, iii). the need for a CIR extrapolation scheme, due to the use of a large "decision depth" (usually greater than 5 symbols), especially at high vehicle speeds (&gt;60 Kmph). In view of the complexity of the computations, requiring more than 5 million manipulations per second (MIPS) to run at a required data rate of 48.6 kilobits per second (Kbps), this MLSE based design limits its use in smaller transceivers.
An alternate MLSE receiver implementation, entailing a computational complexity requiring 9-10 MIPS is disclosed in "Receiver Performance for the North American Digital Cellular System" by G. Larsson, B. Gudmundson, K. Raith, 41st Vehicular Technology Society Conference Proceedings, pp. 1-6, St. Louis, Mo., May 1991. Although this MLSE receiver satisfies the requirements of U.S. TDMA digital cellular telephone system, the complexity is considered to be too high for implementation in a mobile radio receiver. It is, however, a good candidate equalizer for the base station.
U.S. patent application "Adaptive Maximum Likelihood Demodulator" by Paul W. Dent Ser. No. 07/868,339 describes a method of demodulating digital information employing a maximum likelihood sequence estimator (MLSE) equalizer.
Currently there is a need for a simplified digital radio equalizer for demodulating information encoded in a transmitter in accordance with the U.S. digital cellular standard (IS-54) and more generally, a need for realizing a low complexity MLSE equalization technique for TDMA digital radio transmissions over channels producing ISI.